The physiological role of Escherichia coli azoreductase AzoR was studied. It was found that AzoR was capable of reducing several benzo-, naphtho-, and anthraquinone compounds, which were better substrates for AzoR than the model azo substrate methyl red. The ⌬azoR mutant displayed reduced viability when exposed to electrophilic quinones, which are capable of depleting cellular reduced glutathione (GSH). Externally added GSH can partially restore the impaired growth of the ⌬azoR mutant caused by 2-methylhydroquinone. The transcription of azoR was induced by electrophiles, including 2-methylhydroquinone, catechol, menadione, and diamide. A transcription start point was identified 44 bp upstream from the translation start point. These data indicated that AzoR is a quinone reductase providing resistance to thiol-specific stress caused by electrophilic quinones.

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Quinone reductase activity of azoreductase AZR from Rhodobacter sphaeroides was reported. High homologies were found in the cofactor/substrate-binding regions of quinone reductases from different domains. 3D structure comparison revealed that AZR shared a common overall topology with mammal NAD(P)H/quinone oxidoreductase NQO1. With menadione as substrate, the optimal pH value and temperature were pH 8-9 and 50 degrees C, respectively. Following the ping-pong kinetics, AZR transferred two electrons from NADPH to quinone substrate. It could reduce naphthoquinones and anthraquinones, such as menadione, lawsone, anthraquinone-2-sulfonate, and anthraquinone-2,6-disulfonate. However, no activity was detected with 1,4-benzoquinone. Dicoumarol competitively inhibited AZR's quinone reductase activity with respect to NADPH, with an obtained K (i) value of 87.6 microM. Significantly higher survival rates were obtained in Escherichia coli YB overexpressing AZR than in the control strain when treated by heat shock and oxidative stressors such as H(2)O(2) and menadione.

Magnetically recoverable noble metal nanoparticles are promising catalysts for chemical reactions. However, the chemical synthesis of these nanocatalysts generally causes environmental concern due to usage of toxic chemicals under extreme conditions. Here, Pd/Fe3O4, Au/Fe3O4 and PdAu/Fe3O4 nanocomposites are biosynthesized under ambient and physiological conditions by Shewanella oneidensis MR-1. Microbial cells firstly transform akaganeite into magnetite, which then serves as support for the further synthesis of Pd, Au and PdAu nanoparticles from respective precursor salts. Surface-bound cellular components and exopolysaccharides not only function as shape-directing agent to convert some Fe3O4 nanoparticles to nanorods, but also participate in the formation of PdAu alloy nanoparticles on magnetite. All these three kinds of magnetic nanocomposites can catalyze the reduction of 4-nitrophenol and some other nitroaromatic compounds by NaBH4. PdAu/Fe3O4 demonstrates higher catalytic activity than Pd/Fe3O4 and Au/Fe3O4. Moreover, the magnetic nanocomposites can be easily recovered through magnetic decantation after catalysis reaction. PdAu/Fe3O4 can be reused in at least eight successive cycles of 4-nitrophenol reduction. The biosynthesis approach presented here does not require harmful agents or rigorous conditions and thus provides facile and environmentally benign choice for the preparation of magnetic noble metal nanocatalysts.